1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to methods of simultaneously laser shock peening opposite sides of an article using offset laser beams at oblique angles to the surfaces and to articles having simultaneously laser shock peened spots with offset centers on opposite sides of an article.
2. Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477 entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957 entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No.: 5,756,965 entitled xe2x80x9cOn The Fly Laser Shock Peeningxe2x80x9d; U.S. Pat. No. 5,591,009 entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,531,570 entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447 entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329 entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328 entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP""D, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shock wave into the component. This compressive shock wave caused by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the dynamic modules of the material. Dual sided simultaneous laser shock peening includes simultaneously striking both sides of an article by two laser beams in order to increase the compressive residual stress in the material. The laser beams are typically balanced in order to minimize material distortion. The initial compressive waves pass through the material from each of the sides and are reflected back from the interface of the two initial compressive waves. The reflected waves turn into a tension wave. The combined tensile stress of the reflected waves, when the reflected tension waves from the both sides meet at mid-point in the same axial direction, can be greater than the strength that the material can handle and a crack can be initiated at the mid-plane where the two shock waves meet.
Another characteristic of LSP that limits its engineering effectiveness is the formation of deleterious release waves that create tensile strains. The released waves may form spontaneously following the compressive front or may result from reflection at a surface with impedance mismatch such as at the outer surface of a component being laser shock peened. When multiple release waves are simultaneously propagating in a component, they may add in a manner termed superposition. This superposition of tensile waves may reduce the effectiveness of the beneficial compressive strains or may even cause tensile fracture within the component. This superposition of the two spatially concentric waves thus reduces the beneficial effects which may be measured by HCF testing.
U.S. Pat. No. 5,492,447 discloses laser shock peening an interior annular region by orbiting a laser beam at an oblique angle to the interior surface of the interior annular region. U.S. Pat. No. 5,911,890 teaches controlling the incident angle of the laser beam applied to the workpiece and controlling the shape of the beam with lenses, polarizers, and particular transparent overlay geometries. The apparatus and methods disclosed includes use of structure for controlling the position and incident angle of the laser beam and controlling the polarization and/or the shape of the incident impact area, based on such incident angle. The patent teaches that an oblique incident angle laser beam having a circular cross-section causes the shape of the impact spot to be elliptical and that the consequences of such a change of the incident spot shape necessarily changes the energy density applied to the workpiece. The patent further teaches that the energy density per unit area compared to other areas on the same surface creates a possibility of non-uniformly working the material, thereby, possibly losing some of the benefits of laser shock peening. This non-uniformity of energy application to a workpiece may cause severe problems, particularly, when hitting a workpiece from opposite sides at the same time, as used with a split beam laser system. Such opposing hits are sometimes needed on workpieces of thin cross-section, such as disks, blades, and other workpieces of different geometries. In conventional split beam processing, there is a possible effect of not having the laser processed portions on the opposite sides of the workpiece worked identically, and at the same time when elliptical spots are utilized. Such non-uniform working of the workpiece may cause over or under working of the material or distortion of the workpieces, thereby, not achieving the goals of laser shock processing. Furthermore, based upon the oblique angle along with the particular transparent overlay material utilized, polarization issues regarding the reflection of the laser beam from the surface of the transparent overlay layer can possibly degrade and reduce the energy applied to the workpiece. Thus, in general, the patent concludes not to use elliptical laser spots but rather a more complicated apparatus and method to significantly reduce the non-uniformity of the applied energy to a workpiece by modifying the shape of the applied laser energy pulse. The patent teaches to pass the pulse of energy through a lens to reform the shape of the incident area on the workpiece, to counteract geometric effects created by the workpiece surface orientation on the incident area shape. Oblique laser beams are very useful for laser shock peening blades edges of an integrally formed bladed rotor sections of what is often referred to as an integrally bladed rotor (IBR) or an integrally bladed disk (BLISK) having two or more spaced apart rows of blades integrally mounted or formed on a drum rotor or disk.
Thus, it is highly desirable to have a process for and to produce an article that is simultaneously laser shock peened on two opposite sides using oblique laser beams and eliminate the mid-plane cracks by lowering the combined tensile stress of the reflected waves just below the maximum or allowable tensile stress of the material. It is also highly desirable to be able to eliminate or reduce loss of HCF benefits or effectiveness of the beneficial compressive strains from laser shock peening caused by the superposition of tensile waves.
The invention is a method for laser shock peening first and second surfaces on opposite first and second sides respectively of an article by simultaneously firing first and second laser beams at first and second oblique angles with respect to the first and second surfaces so as to form overlapping adjacent laser spots on the surfaces. The first and second laser beams are fired such that first and second centerlines of the first and second laser beams impinge the first and second surfaces at first and second laser beam centerpoints through which pass first and second axes that are substantially normal to the first and second surfaces at the first and second laser beam centerpoints, respectfully. The first and second axes are offset and the laser beams are fired with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the article from the surfaces. In another embodiment, the first and second laser beams are fired such that first and second centerlines of the first and second laser beams impinge the first and second surfaces at first and second laser beam centerpoints that are longitudinally spaced apart and transversely offset from each other. The first and second spots may be substantially parallel. The laser beams may be aimed and fired in a manner to produce first and second patterns on the first and second surfaces of the article having overlapping adjacent rows of overlapping adjacent ones of the first and second spots, respectively. The first and second patterns may be formed by continuously moving the article while holding stationary and continuously firing the laser beams with repeatable pulses with relatively constant periods between the pulses wherein the first and second surfaces are laser shock peened using sequences wherein each sequence comprises continuously moving the article while continuously firing the stationary laser beams on the surfaces such that on each of the surface portions adjacent ones of the laser shock peened spots are hit in different ones of the sequences in the set. The method may include coating the surface portions with an ablative coating before and in between the sequences in the set. Offset first and second laser beam centerpoints that are longitudinally spaced apart and transversely offset from each other is disclosed in U.S. patent application Ser. No. 09/438,513 and oblique laser beams are disclosed in U.S. patent application Ser. No. 09/771,856 filed Jan. 29, 2001 both of which are assigned to the present assignee of this patent.
In a first exemplary embodiment of the invention, the invention is used for laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blade with circular cross-section oblique laser beams, firing the laser beams at oblique angles with respect to the surfaces so as to form elliptical shaped laser spots on the surfaces, overlapping adjacent elliptical shaped laser spots, and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces. The elliptical shaped laser spots have major axis extending away from the edge and over the surfaces and transverse minor axis and, in a more particular embodiment of the invention, the elliptical shaped laser spots overlap by about 50% and the laser spots extend over the edge. Relative movement between the laser beams and the surfaces is effected while the laser beams are being fired.
A second exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element circumscribed about an axis of rotation and having an annular space between adjacent axially spaced apart forward and aft and rows of blades. This method laser shock peens leading or trailing edges that border the space by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blades in one of the rows with circular cross-section laser beams, firing the laser beams at a first oblique angle with respect to the surfaces so as to form elliptical shaped laser spots on the surfaces and at a second oblique angle with respect to the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, and overlapping adjacent elliptical shaped laser spots and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces.
A third exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blade with circular cross-section first and second laser beams respectively, firing the first laser beam at an oblique angle with respect to the pressure side surface so as to form elliptical shaped laser spots on the pressure side surface, firing the second first laser beam at about a normal angle with respect to the suction side surface so as to form circular shaped laser spots on the suction side surface, and overlapping adjacent elliptical shaped laser spots and circular shaped laser spots respectively and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces.
A fourth exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element circumscribed about an axis of rotation and having an annular space between adjacent axially spaced apart forward and aft rows of blades, wherein the edges being laser shock peened border the space. This method laser shock peens leading or trailing edges that border the space by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blades in one of the rows with circular cross-section laser beams, firing the first laser beam at a first oblique angle with respect to the pressure side surface so as to form elliptical shaped laser spots on the pressure side surface and at a second oblique angle with respect to the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, firing the second first laser beam at about a normal angle with respect to the suction side surface so as to form circular shaped laser spots on the suction side surface and at a second oblique angle with respect to the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, and overlapping adjacent elliptical shaped laser spots and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces. Low energy laser beams, on the order of 3-10 joules or perhaps even in a range of 1-10 joules may be used. The use of low energy laser beams of this order of magnitude is disclosed in U.S. Pat. No. 5,932,120, entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d, which issued Aug. 3, 1999 and is assigned to the present assignee of this patent and is incorporated herein by reference.
The present invention may lower the cost, time, man power and complexity of performing laser shock peening by allowing crack free dual sided simultaneous laser shock peening. The present invention provides a dual sided simultaneous laser shock peening method which is able to reduce or eliminate mid-plane cracks by lowering the combined tensile stress of the reflected waves below the maximum or allowable tensile stress of the material. The invention provides a simultaneously dual sided laser shock peened article with fewer or no mid-plane cracks. The invention can be used to eliminate or reduce loss of HCF benefits or effectiveness of the beneficial compressive strains from laser shock peening caused by the superposition of tensile waves. The invention has been found useful to provide a positive effect on HCF capability of laser shock peened articles and, in particular, laser shock peened leading edges of airfoils gas turbine engine blades and vanes.
The present invention is a faster and more cost efficient method to laser shock peen surfaces of articles such as leading and trailing edges gas turbine engine blades mounted on rotor sections or elements using oblique laser beams. An integrally formed bladed rotor section is also referred to as an integrally bladed rotor (IBR) or integrally bladed disk (BLISK) having two or more spaced apart rows of blades integrally mounted or formed on a drum rotor or disk respectively. The blades are designed to operate in high tensile and vibratory stress fields and laser shock peening enables the blades to better withstand fatigue failure due to nicks and tears in the leading and trailing edges of the blades and, therefore, have an increased life over conventionally constructed blades.
The use of low energy lasers allows a laser shock peening production line to be set up and operated less expensively compared to those suggested in the prior art, due to lower capital outlay. The line could be less complex to develop, design, and construct because the method for the present invention uses direct laser beams without intervening lenses for laser beam shaping.